Question about mod developing

[Mnemozin]

In the past i've played a few Diablo 1 mods, namely The Hell.
Sometimes Mordor, creator of that mod, and some other programmers were complaining about how difficult it is to implement some feature(mostly when someone asked them why the feature wasn't already there). That they have to find right part of the code through assembler hacking, figure out what they actually have to do, blah-blah-blah, all that kind of stuff. And here you are, with crafting, high res and zooming, chest with an actual interface, completely new quests(IIRC), etc. I've heard that this is actually a remake, not a mod, but i don't know if that's true. Game uses the same engine as far as i can tell.
The fact that this mod is so small(35mb, really?) makes even less sense to me.

Can someone explain how exactly this mod(or whatever this is) has been created?


p.s. i'm 100% sure that the part about The Hell developers isn't correct, but you got the idea.